1. Field of the Invention
The present invention relates to an apparatus for detecting displacement, for example, to an apparatus for detecting displacement of laser interference type.
2. Description of Related Art
Conventionally, there is known an apparatus for detecting displacement of laser interference type. FIG. 11 shows a conventional laser interference type displacement detection apparatus 100 that includes a main scale 110, and a detection head 120. The main scale 110 has a reflection type diffraction grating 111 along the longitudinal direction thereof or the length measurement direction. The detection head 120 has a light emission/reception unit 130, and an optical device unit 600.
The light emission/reception unit 130 has a light source 131 for emitting a laser light, and a light reception unit 134 for receiving an interfering light reflected by the main scale 110.
The optical device unit 600 has a beam splitter 601 for splitting a light from the light source 131, a first mirror 602 for reflecting one of lights split by the beam splitter 601 toward the main scale 110, a second mirror 603 and a third mirror 604 for reflecting the other of lights split by the beam splitter 601 toward the main scale 110, a fourth mirror 605 for reflecting one of diffracted lights reflected by the main scale 110 toward a half mirror 607, and a fifth mirror 606 for reflecting the other of diffracted lights reflected by the main scale 110 toward the half mirror 607.
In this configuration, a light emitted from the light source 131 is split by the beam splitter 601, and thus split lights are diffracted by the main scale 110. The lights diffracted by the main scale 110 interfere after passing through the half mirror 607, and the interfering light is received by the light reception unit 134. When the main scale 110 is displaced, brightness of the interfering light varies. Thus, displacement of the main scale 110 can be detected from the variation of the brightness. Since a light is split into two, and the split lights diffracted by the main scale 110 are made to interfere, the direction of displacement of the main scale 110 can be detected.
In splitting and reflecting a light, a diffraction grating may be used. A diffraction grating is arranged instead of a beam splitter and mirrors (for example, see Japanese Patent Laid-Open Publication No. 2002-372407).
When splitting a light from the light source 131, reflecting and diffracting thus split lights using the main scale 110, and making thus diffracted lights interfere, many optical components such as the beam splitter 601, mirrors 602 to 606, etc. are required. Furthermore, it takes a lot of trouble to adjust the arrangement and optical axes of the mirrors 601 to 606, etc.
When many optical components are arranged, the apparatus is enlarged and also optical paths are elongated. Since interference of a laser light varies due to variation of atmospheric density, elongated optical paths may lower detection accuracy.
In the case of using a diffraction grating instead of a beam splitter and mirrors, diffracted lights reflected by the main scale have to be return to the main scale so as to increase the number of reflection times to improve detection resolution, therefore reflection mirrors have to be prepared. Therefore, it is impossible to satisfy both of improving detection resolution and reducing the number of optical components to shorten optical paths, concurrently.